Hard fouling is the generic name given to the accumulation of the hydroid class of invertebrate. Examples include barnacles, limpets, mussels and tubeworms. A typical example can be seen in the photograph which shows mussel colonisation of a marine pump.
Mussels in particular are prolific breeders which attach themselves to almost any submerged surface (rocks, boats, piers, pip pees and to each other) by secreting horny sticky threads. Egg fertilisation and production occurs within hours of the water temperature rising above 12 °C. After hatching and as long as the water temperature stays between 14 °C and 24 °C the free swimming larvae will appear in plankton and be dispersed by water currents. Spores can settle out if the circulation velocities are low enough – under 1-1.5 m/s. Once the shell grows too heavy it will attach itself to a suitable substrate by means of its threads. By the end of the first growing season the female mussel can produce 30,000 to 40,000 eggs. Population growth is therefore rapid and will spread quickly through a seawater system where the flow of water ensures a good food supply
Marine growth is therefore a problem in most oceans of the world, including colder environments such as the North Sea. The consequences are of particular concern for submerged pumps and water intakes. Initially resulting in reduced flows and pressure, untreated marine growth can lead to complete blockage of pumps and pipe work. Fire pumps are particularly at risk due to their infrequent operation.